This invention relates generally to plastic transparencies, and, more particularly, to plastic transparencies suitable for use in an acidic environment.
Plastic transparencies comprised primarily of polymethyl methacrylate are used in numerous applications where light weight, strength, and optical clarity are required. One major application of such transparencies is in aircraft. In such aircraft applications, optical hazing due to cracking and crazing of the transparency's exposed surface is a significant problem. Many environmental factors such as humidity are believed to aggravate this problem.
It has recently been theorized that sulfuric acid in the air aggravates the optical hazing problem in such aircraft transparencies. This acid is believed to result from the significant amounts of sulfur dioxide released into the atmosphere by volcanic activity around the world. The sulfur dioxide reacts with water vapor in the air to produce minute droplets of sulfuric acid. The acid droplets adhere to the exposed aircraft surfaces, and may be preferentially attracted to plastic transparency surfaces due to their static charge. The acid acts as a solvent or plasticizer to soften the plastic surface.
Tests have shown that sulfuric acid severely attacks acrylic transparencies presently in general use in aircraft. There is therefore a need for an improved composition for transparencies of this kind, which is more resistant to acid attack. The present invention fulfills this need.